Imbalanced Mach-Zehnder Interferometer Research Articles

Phase Error Evaluation via Differentiation and Cross-Multiplication Demodulation in Phase-Sensitive Optical Time-Domain Reflectometry

Phase-sensitive optical time-domain reflectometry (φOTDR) is a technology for distributed vibration sensing, where vibration amplitudes are determined by recovering the phase of the backscattered light. Measurement noise induces phase errors, which degrades sensing performance. The phase errors, using a differentiation and cross-multiplication (DCM) algorithm, are investigated theoretically and experimentally in a φOTDR system based on a phase retrieval configuration consisting of an imbalanced Mach–Zehnder interferometer (IMZI) and a 3 × 3 coupler. Analysis shows that phase error is highly dependent on the AC component of the obtained signal, essentially being inversely proportional to the product of the power of the light backscattered from two locations. An analytical expression was derived to estimate the phase error and was confirmed by experiment. When applied to the same measurement data, the error is found to be slightly smaller than that obtained using in-phase/quadrature (I/Q) demodulation. The error, however, increases for longer measurement times.

Frequency-scanned microresonator soliton comb with tracking of the frequency of all comb modes.

Rapid and large scanning of a dissipative Kerr-microresonator soliton comb with characterization of all comb modes along with the separation of the comb modes is imperative for the emerging applications of frequency-scanned soliton combs. However, the scan speed is limited by the gain of feedback systems, and measurement of the frequency shift of all comb modes has not been demonstrated. To overcome the limitation of the feedback, we incorporate feedback with feedforward. With an additional gain of >40dB by a feedforward signal, a dissipative Kerr-microresonator soliton comb is scanned by 70 GHz in 500µs, 50 GHz in 125µs, and 25 GHz in 50µs (= 500 THz/s). Furthermore, we propose and demonstrate a method to measure the frequency shift of all comb modes, in which an imbalanced Mach-Zehnder interferometer with two outputs with different wavelengths is used. Because of the two degrees of freedom of optical frequency combs, the measurement at two different wavelengths enables estimation of the frequency shift of all comb modes.

Direct detection based φOTDR using the Kramers-Kronig receiver.

A Kramers-Kronig (KK) receiver is applied to a phase-sensitive optical time domain reflectometry based on direct detection. An imbalanced Mach-Zehnder interferometer with a 2×2 coupler is used in sensing system to encode the phase information into optical intensity. The directly obtained signal is treated as the in-phase component, and the KK receiver provides the quadrature component by Hilbert transform of the obtained signal, so that the optical phase can be retrieved by IQ demodulation. The working principle is well explained, and the obtained phase variance is theoretically analyzed. The experiment demonstrates the functionality of the sensor and validates the theoretical analysis.

Evaluating Phase Errors in Phase-Sensitive Optical Time-Domain Reflectometry based on I/Q Demodulation

The phase errors resulting from in-phase/quadrature (I/Q) demodulation in phase-sensitive optical time-domain reflectometry (φOTDR) are investigated theoretically, numerically, and experimentally for two phase recovery schemes: one employing Kramers–Kronig (KK)-based coherent detection, and a second one based on a direct-detection scheme using an imbalanced Mach–Zehnder interferometer, a 3 × 3 coupler and 3 photodetectors. Mathematical models for the estimation of the differential phase variance are proposed and experimentally validated for these two schemes. The probability density function of the phase variance verifies an uneven longitudinal distribution of the phase errors in the two cases. Results point out that KK-based φOTDR sensors have no special advantages in terms of noise performance compared to standard full I/Q coherent detection, so that large phase errors can be verified at fading locations, leading to severe phase unwrapping problems. On the contrary, the direct-detection scheme based on interferometer is highly dependent on the signal difference between photodetectors, showing higher robustness against fading even if its probability density function is wider than the KK-based case. Experimental results also show the advantages of selecting data points with high in-phase and quadrature amplitudes to achieve reliable vibration measurements, especially in KK-based φOTDR sensors.

Time-Stretch Spectroscopy Based on Laser Cavity Tuning With a Dual-Function Delay Line

Time-stretch spectroscopy has established as a viable technique with high resolution and single-shot capability. However, it generally requires ultra-wideband photodetectors and electronics, and its performance is often restricted by a fundamental tradeoff between spectral resolution and signal-to-noise ratio. Here, we report a time-stretch spectrometer based on the concept of optical sampling by laser cavity tuning. A kilometer-scale dual-function fiber-optic link serves as both a pulse stretcher and a long delay line in an imbalanced Mach–Zehnder interferometer. Measurement results based on several device and atomic samples are presented. Theoretical forms of the total spectral range and spectral resolution are derived and compared with experimental data. A spectral resolution as small as 8 GHz has been achieved. Resolution-limiting factors are discussed and possible solutions are presented.

10 kHz linewidth mid-infrared quantum cascade laser by stabilization to an optical delay line.

We present a mid-infrared quantum cascade laser (QCL) with a sub-10kHz full width at half-maximum linewidth (at 1s integration time) achieved by stabilization to a free-space optical delay line. The linear range in the center of a fringe detected at the output of an imbalanced Mach-Zehnder interferometer implemented with a short free-space pathlength difference of only 1m is used as a frequency discriminator to detect the frequency fluctuations of the QCL. Feedback is applied to the QCL current to lock the laser frequency to the delay line. The application of this method in the mid-infrared is reported for the first time, to the best of our knowledge. By implementing it in a simple self-homodyne configuration, we have been able to reduce the frequency noise power spectral density of the QCL by almost 40dB below 10kHz Fourier frequency, leading to a linewidth reduction by a factor of almost 60 compared to the free-running laser. The present limits of the setup are assessed and discussed.

Coarse wavelength division (de)multiplexer using an interleaved angled multimode interferometer structure

We have demonstrated a coarse wavelength (de)multiplexing structure on the silicon-on-insulator platform. It comprises two 4-channel angled multimode interferometers interleaved with an imbalanced Mach-Zehnder interferometer (MZI) leading to an 8-channel multiplexing device. The device requires only single lithography and etching steps for fabrication and has a good tolerance to fabrication errors in terms of waveguide width. The insertion loss and crosstalk achieved are 3-4 dB and –(15–20) dB, respectively. Potential is shown for achieving improved performance using larger waveguide bending radii in the MZI arms and/or (a) local heater(s) for refractive index tuning.

Imbalanced Mach-Zehnder interferometer integrated in micromachined silicon substrate for pressure sensor

This paper describes an interferometric pressure sensor designed to work as a remote sensor in a coherence modulation scheme, allowing a linear phase read-out of the signal. The sensor is realized from a silicon nitride waveguide Mach-Zehnder interferometer integrated on a (100) silicon substrate, and includes a micromachined membrane below a branch of the Mach-Zehnder. The characterization is achieved by a spectral analysis and shows a nonlinear relationship between the static pressure applied to the membrane and the phase variation thus obtained. The analytical description takes into account the induced deflection of the membrane and the resulting mechanical elongation of the waveguide.

Double security level in a telecommunication system based on phase coding and false data transmission

A new communication system exhibiting a double security level is demonstrated. Information coding is based on relative phase changes between consecutive time intervals of the light emitted from a highly coherent CW source. An imbalanced Mach-Zehnder interferometer whose time delay matches the intervals duration is used to decode the data. The first security level is related to the constant optical power in the transmission link. To further enhance the security, a low coherent source is used to send false data down the transmission link. Common mode rejection is used to separate the real signal from the false signal. This system provides a high signal-to-noise ratio as well as a low crosstalk in a wavelength division multiplexing network.